Unreadable text during distupgrade: follow-up

[ Impact ]

This bug is a follow-up of bug #2034986. Please see that bug, especially comment #54, for a more verbose background to this bug report.

The short story:

* Ubuntu Cinnamon is hit by 'the unreadable text issue' described in the other bug.

* Several Ubuntu flavors fail to reset the desktop font after reboot, since they lack the systemd integration needed for a user service to work.

The proposed changes include

* a correction of the applied gsettings schema for Ubuntu Cinnamon

* use of an autostart file instead of a systemd service for a bunch of flavors.

[ Test Plan ]

Prepare updated 23.04 versions of Ubuntu Cinnamon and Ubuntu Desktop.

Install version 1:23.04.7 of ubuntu-release-upgrader-core (may be in lunar-proposed)

For upgrading use: "update-manager -d -p"
(not sure if '-d' is still needed)

As regards Ubuntu Cinnamon:

* Confirm that the text is readable during the whole upgrade.

* Confirm after reboot that the command

  gsettings get org.cinnamon.desktop.interface font-name

  now outputs 'Ubuntu 11' and not the temporary 'Sans 11'.

As regards Ubuntu Desktop simply confirm that the upgrade works as intended and that you don't notice any regression.

[ Where problems could occur ]

I might have made some mistake as regards gsettings schemas and/or flavors which need autostart. But the code it straightforward, and I dare to say that the _set_generic_font() function works as intended for more flavors with the proposed changes. And if there would be a mistake with respect to some particular flavor, it would affect that flavor only.

[ Other Info ]

It's worth mentioning (here too) that the applied model for preventing the readability issue at upgrade, i.e. changing a dconf value which represents the desktop font, is not effective in the case of Kubuntu, Lubuntu and Xubuntu. They are just not hit by the issue since the fonts they use have not changed during the cycle.